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Must-know facts
20 facts
- 1Virchow (1855) added 'Omnis cellula-e cellula' to the Schleiden (1838) and Schwann (1839) cell theory.
- 2Leeuwenhoek was the first to see and describe a live cell; the nucleus was discovered by Robert Brown (1831).
- 3Smallest cells: Mycoplasmas. Largest isolated single cell: ostrich egg. Human RBC: about 7.0 μm in diameter; bacteria 3–5 μm.
- 4Mycoplasma is the one prokaryote without a cell wall.
- 5Bacterial envelope: glycocalyx (slime layer if loose, capsule if thick and tough), cell wall, plasma membrane.
- 6Mesosome functions: cell wall formation, DNA replication and distribution, respiration, secretion, more surface area and enzymes.
- 7Prokaryotic ribosomes are 70S (50S + 30S), and mitochondria and chloroplasts also carry 70S ribosomes; eukaryotic cytoplasmic ribosomes are 80S (60S + 40S).
- 8Human RBC membrane: about 52% protein, 40% lipid. Fluid mosaic model: Singer and Nicolson, 1972.
- 9Na⁺/K⁺ pump is active transport and uses ATP.
- 10Cell wall layers outward: secondary wall, primary wall, middle lamella (calcium pectate).
- 11Endomembrane system = ER + Golgi + lysosomes + vacuoles; mitochondria, chloroplasts and peroxisomes are excluded.
- 12SER makes lipids and, in animal cells, steroidal hormones; RER is continuous with the outer nuclear membrane.
- 13Golgi: discovered by Camillo Golgi (1898); cisternae 0.5–1.0 μm; cis = forming (convex) face, trans = maturing (concave) face; site of glycoprotein and glycolipid formation.
- 14Lysosomes are packaged by the Golgi and hold hydrolases (lipases, proteases, carbohydrases) that work best at acidic pH.
- 15Vacuole membrane is the tonoplast; the vacuole can fill up to 90% of a plant cell.
- 16Mitochondria: 0.2–1.0 μm wide (average 0.5 μm), 1.0–4.1 μm long; matrix has circular DNA, some RNA and 70S ribosomes; divide by fission.
- 17Leucoplasts: amyloplast = starch, elaioplast = oils and fats, aleuroplast = proteins.
- 18Chloroplasts: 5–10 μm long, 2–4 μm wide; one per cell in Chlamydomonas, 20–40 per mesophyll cell.
- 19Axoneme is 9+2 (doublets); centriole is nine peripheral triplets with a central hub and no central microtubules.
- 20Nucleolus is not membrane-bound and is the site of active rRNA synthesis; perinuclear space is 10–50 nm.
Common traps
Where marks are lost
Crediting Schleiden and Schwann with the idea that new cells come from pre-existing cells.
That was Virchow in 1855. Schleiden and Schwann said organisms are made of cells; they could not explain how new cells form.
Adding subunit S values: assuming 50S + 30S should make an 80S ribosome.
S values are sedimentation coefficients, not masses, so they do not add. 50S + 30S = 70S; 60S + 40S = 80S.
Putting mitochondria or chloroplasts in the endomembrane system because they have membranes.
Membranes are not the test; coordinated function is. Only ER, Golgi, lysosomes and vacuoles make up the endomembrane system.
Placing the secondary wall outside the primary wall.
The secondary wall is laid down later on the inner side, towards the plasma membrane. Middle lamella is the outermost shared layer.
Treating the centriole and the axoneme as the same 9+2 structure.
Axoneme: nine doublets plus a central pair (9+2). Centriole: nine triplets around a proteinaceous hub, no central microtubules.
Calling the nucleolus a membrane-bound organelle.
The nucleolus has no membrane; its material is continuous with the nucleoplasm.
Mixing up the three leucoplasts.
Amylo = starch (think amylase), elaio = oil (think olive), aleuro = protein.
Assuming every cell has a nucleus or that every prokaryote has a wall.
Mature RBCs of many mammals and sieve tube cells lack a nucleus; Mycoplasma lacks a cell wall.
Swapping cis and trans faces of the Golgi.
Cis is the forming face that receives ER vesicles (convex); trans is the maturing face that releases products (concave).
Thinking plant cells lack centrioles and animal cells lack vacuoles, so neither can ever occur.
Centrioles are absent from almost all plant cells, not all; and although only plant cells have the large central vacuole, other cells have smaller ones, such as the contractile vacuole of Amoeba and the food vacuoles of many protists.
Key terms
20 terms
- Prokaryotic cell
- A cell whose genetic material lies free in the cytoplasm with no nuclear envelope and which has no membrane-bound organelles.
- Plasmid
- A small circular DNA outside the bacterial chromosome that carries extra traits such as antibiotic resistance.
- Mesosome
- An inward fold of the bacterial plasma membrane that helps with wall formation, DNA handling, respiration and secretion.
- Glycocalyx
- The outermost layer of the bacterial envelope, called a slime layer when loose and a capsule when thick.
- Polysome
- A string of ribosomes translating the same mRNA at the same time.
- Inclusion bodies
- Membrane-free stores of reserve material in prokaryotic cytoplasm, such as glycogen or phosphate granules.
- Fluid mosaic model
- The Singer–Nicolson picture of the membrane as a quasi-fluid lipid bilayer with proteins that can drift sideways.
- Middle lamella
- The calcium pectate layer that cements adjacent plant cells together.
- Plasmodesmata
- Cytoplasmic channels passing through plant cell walls that connect neighbouring cells.
- Endomembrane system
- The functionally linked set of ER, Golgi apparatus, lysosomes and vacuoles.
- Cisternae
- The flat, stacked, disc-like sacs that make up the Golgi apparatus.
- Tonoplast
- The single membrane surrounding the plant vacuole.
- Cristae
- Folds of the inner mitochondrial membrane that increase its surface area.
- Grana
- Stacks of thylakoids inside the chloroplast, linked by stroma lamellae.
- Svedberg unit (S)
- The unit of sedimentation coefficient, an indirect measure of a particle's size and density.
- Axoneme
- The microtubule core of a cilium or flagellum, arranged in a 9+2 pattern.
- Basal body
- A centriole-like structure at the base of a cilium or flagellum from which it grows.
- Kinetochore
- A disc-shaped structure on either side of the centromere.
- Satellite
- A small chromosome segment set apart by a secondary constriction.
- Microbodies
- Small membrane-bound vesicles containing various enzymes, present in plant and animal cells.